1. Field of the Invention
The present invention relates to a method for controlling the machining of mechanical pieces in a machine tool, like a grinding machine, comprising the in-process measuring of the pieces in order to obtain a first measurement signal; the control of the machine tool as a function of the first measurement signal; the post-process measuring of the pieces in order to obtain a second measurement signal; and the correction of the first measurement signal depending on the value of the second measurement signal.
The invention also relates to an apparatus for carrying out said method.
2. Description of the Prior Art
It is known to control the machining of mechanical pieces in machine tools, more specifically in grinding machines, by means of one or more electronic measuring devices for measuring the dimensions of a piece while it is being machined ("in-process").
Generally, the measuring device consists of a comparator gauge that determines the deviation of a piece dimension from a nominal dimension with respect to which the gauge has been previously set to zero. For example, with reference to an external cylindrical grinding machine, the gauge determines the deviation of a piece outside diameter from a nominal value. As prefixed deviation values are met, the gauge controls the switching from a coarse grinding phase to a fine finishing phase, and then to a "sparkout" phase (machining without any radial feed occurring between the grinding wheel and the piece, whereby there is an--at least partial--elimination of the previously accumulated deformations and strains), and finally, when the nominal diameter size is reached, the grinding wheel withdrawal from the piece.
In general, the gauge is retracted by a slide, in order to facilitate the unloading of a machined piece and the loading of a new piece.
It is also known to arrange a master or standard piece in the machine so that, when the slide retracts the gauge, the latter gauges the master, so as to detect any possible deviations from the zero-setting condition. These deviations can be used to correct the signal provided by the gauge during the in-process operation.
However, even under the circumstance that the in-process gauge always remain perfectly zero-set against the master piece, the final dimensions of the machined pieces might vary because, for example, the working conditions and the machine response time are subject to drifts and other alterations. It is also known, to correct the measurement signal provided by an in-process gauge, i.e. the zero-setting of the in-process gauge, by means of a "post-process" gauge. The post-process gauge is installed at the machine output, in order to check the machined piece after it has been unloaded from the working position and placed in a post-process measuring station.
The post-process gauge usually provides more accurate and repeatable measurements, with respect to an in-process one, because the post-process measuring station is unaffected by disturbances originated by the grinding wheel pressure against the piece, the grinding wheel rotation, the coolant, etc. and is affected to a less extent by the variations in the piece temperature. In fact, the temperature of the pieces at the post-process station varies within a more restricted range with respect to the variations occurring during the machining operation; even the parts of the gauge, like in particular the movable arms of directly contacting gauges, in the post-process applications cause minor thermal drifts.
The main advantages in the use of the post-process gauges consist--apart from the high accuracy and repeatability and the possibility of compensating, at least partially, any variations in the functioning of the grinding machine--in the elimination or at least the reduction of the periodic operations for testing the zero-setting conditions of the in-process gauge with respect to a master piece, arranged either on or off the machine. Consequently, the down-times of the grinding machine are reduced.
However, the use of one or more post-process gauges obviously implies considerable additional expenses, due to the provision of a post-process gauge itself (measuring head and associated amplifier), of a measuring station (mechanical references for the piece) and to a more complex piece handling.